This invention relates to a method for recovering oil from a subterranean formation by a waterflood and more particularly relates to a method for accelerating the recovery of oil by the waterflood process.
Since only a portion of the original oil in a subterranean oil-containing formation is produced through primary recovery, i.e., where the initial formation energy, supplemented by gas lifting, gas repressurization and mechanical pumping, if necessary, is used to recover the crude oil, it is necessary to utilize secondary recovery techniques to increase the ultimate recovery of oil.
The most economical and widely practiced secondary recovery technique is waterflooding, where an aqueous fluid is introduced into the subterranean formation through injection wells to drive oil through the formation to offset production wells.
Much work has been done on improving the efficiency of the waterflood process through the use of additives, such as surface-active agents or surfactants. It is thought that after a typical waterflood process, much of the oil is left behind in the formation pore spaces as discrete droplets which, because of the high interfacial tension between the injected water and the formation oil, are unable to deform and flow through the flow channels which link the pore spaces in the formation. It is thought that the addition of surface-active agents or surfactants lowers the interfacial tension between the injected water and reservoir oil, which in turn allows the oil droplets to deform and flow through the flow channels linking the pore spaces and thereby flow to the production wells. U.S. Pat. No. 4,293,428 is directed to the injection of certain propoxylated and ethoxylated sulfate, sulfonate, phosphate or carboxylate surfactants in high salinity formations, with the subsequent use of a driving fluid to displace oil from the formation and to recover the displaced oil.
Much work has also been done on improving the injectivity of water injection wells, i.e., the ability to inject water into an oil-containing subterranean formation through a wellbore, by the removal of residual oil from the formation pore spaces in the near wellbore area. An increase in injectivity, while not resulting in any increase in the amount of oil recovered, does serve to accelerate the rate of recovery of oil, thereby improving the economics of the waterflood process. U.S. Pat. No. 3,670,819 is directed to such process. In this patent, the use of an oil external micellar slug followed by a micellar slug of higher water content serves to remove the residual oil and organic deposits from around the injection well, thereby improving the water injectivity of the injection wells.
There is a substantial and unfulfilled need for alternative techniques to improve the injectivity of water injection wells to be used in certain waterflood processes where there is a low formation permeability to water as compared to the formation absolute permeability due to the presence of residual oil in the formation pore spaces.